1. Field of the Invention
The present invention relates to an exhaust emission purifying apparatus for an engine (to be referred to as exhaust emission purifying apparatus hereunder), for reductively removing, using a reducing agent, nitrogen oxides (NOx) in the exhaust gas or emission (it will be referred to as “exhaust emission” hereinafter throughout the description and claims), which is exhausted from the engine, and in particular, to a technology for reducing an offensive odor which is produced when a storage tank storing the reducing agent is opened or closed.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2000-27627 disposes an exhaust emission purifying apparatus which proposes a catalyst purifying system to remove NOx contained in the engine exhaust emission. In the disclosed exhaust emission purifying apparatus, a reduction catalyst disposed in an engine exhaust system, and a reducing agent is injected to be supplied to the upstream side of the reduction catalyst, so that NOx in the exhaust emission and the reducing agent are subjected to the catalytic-reduction reaction, to conduct purifying processing for converting the NOx into harmless components.
The reducing agent is stored in a storage tank at room temperature and in a liquid state, and the reducing agent of necessary amount corresponding to engine operating conditions is injected from an injection nozzle. Further, for the reduction reaction, ammonia having the good reactivity with NOx is used, and as the reducing agent, an aqueous urea solution is used, which is hydrolyzed by the exhaust heat and by the water vapor in the exhaust emission to easily generate ammonia.
However, according to the above-described exhaust emission purifying apparatus, if the temperature of the storage tank becomes higher due to a change in the ambient temperature and so on, the aqueous urea solution in the storage tank undergoes a chemical reaction, and ammonical gas or gases might be produce to fill an upper space of the storage tank. Then, for example when the storage tank is to be replenished with the urea aqueous solution, if an operator detaches a protective cap of the storage tank, there might occur such a situation that that the filled ammonical gas comes to leak toward the outside, to thereby produce the offensive odor. Such offensive odor will be produced not only when the aqueous urea solution is used as the reducing agent but also when a diesel oil which mainly contains the aqueous ammonia solution, hydrocarbon or the like, is used.